Tag: Melt inclusions

The 2014–2015 Holuhraun eruption in Iceland was the largest volume eruption on the island since the 1783–1784 Laki erution (e.g., Neave et al., 2013; 2017), and was one of the most closely monitored eruptions ever to have taken place (Gudmundsson et al., 2016). In this paper, lead by Margaret Hartley and Enikö Bali (who also recently published an associated paper), we present melt inclusion data from a suite of samples collected throughout the eruption.

Variability in melt inclusion compositions indicates that the erupted magma evolved from diverse primary melts by concurrent mixing and crystallisation. Using a refined method of olivine–plagioclase–augite–melt (OPAM) barometry, we place this evolution at mid-crustal depths, in agreement with geophysical indicators of magma storage. Re-equilibration of melt inclusion H2O contents indicates that crystals spent at least 1–12 days in their carrier liquid before eruption, consistent with lateral transport in a mid-crustal dyke from the Bárðarbunga central volcano to the eruption site.

A conceptual model for the entrapment of melt inclusions erupted during the 2014–2015 Holuhraun eruption. Figure from Hartley et al. (2018).

Olivine is almost ubiquitous in primitive basalts, making it an excellent tool for investigating early phases of magmatic evolution and mantle melting conditions. For example, studies of olivine-hosted melt inclusions have provided crucial insights into primitive melt variability, deep magma mixing (e.g., Neave et al., 2013) and the behaviour of volatiles during magma transport from the mantle to the surface (e.g., Neave et al., 2012; 2014). It has also been proposed that the compatible trace-element (CTE) content of olivines themselves provides information about lithological heterogeneity in the mantle (e.g., Sobolev et al., 2007), though the causes of such CTE variability remain highly debated (e.g., Matzen et al., 2017).

In this paper with Oliver Shorttle and Martin Oeser, I present both CTE and incompatible trace-element (ITE) data from primitive Icelandic olivines that we use check the validity of melt inclusion records and investigate causes of geochemical variability in olivine macrocrysts themselves. We demonstrate that olivine macrocrysts are capable of preserving similar patterns of compositional variability to melt inclusions on intra- and inter-eruption lengthscales, and may allow degrees of magma enrichment to be reconstructed in samples where matrix glasses are degraded or absent.

An X-ray map of P in an olivine from the Stapafell eruption. Almost no P zoning can be observed in the olivine; boundary layer crystallisation seems unimportant. This image is approximately 1 mm across.

Although olivines from our enriched case study eruption, Stapafell, are slightly richer in Ni than those from our depleted case study eruption, Háleyjabunga, the CTE content of both eruptions are wholly consistent with melt supply from a peridotitic source. However, independent constraints from the combined major and trace element systematics of Icelandic basalts indicate that enriched melts come from a modally enriched source (Shorttle & Maclennan, 2011); enriched Icelandic basalts are too rich in iron to be derived by melting of depleted mantle. We therefore conclude that enriched domains in the Icelandic mantle are composed of modally enriched peridotite not pyroxenite, and that olivine CTE contents provide an incomplete picture of lithological heterogeneity in the mantle.

Geochemcial records of mantle processes are progressively degraded as magmas differentiate and ascend towards the Earth’s surface. This degradation is particularly severe in the case of volatiles (H2O, CO2, F, S and Cl) that decouple from melts upon reaching vapour saturation. Melt inclusions – pools of silicate liquid that are partially insulated from changes in the external magmatic enronment by their host crystals – are thus appealing targets for investigating the behvaiour of magmatic volatiles. Although numerous recent studies have critically evaluated the effects of syn- and post-entrapment modification on olivine-hosted melt inclusion compositions, little comparable information is available for plagioclase-hosted systems, depite plagioclase’s abundance in mafic magmas.

In order to address this imbalance in undertanding between olivine-hosted and plagioclase-hosted systems, we present volatile and light lithophile element analyses from a large number of mainly plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series from Iceland. Major and trace element data have already been presented in study into the pre-eruptive evolution and storage of the tephra series (Neave et al., 2015).

Volatile-trace element systematics in matrix glasses and melt inclusions used to distinguish between pre-, syn- and post-entrapment signals of variability. Figure from Neave et al. (2017).

The uniformly low CO2 content of melt inclusions cannot be explained by either shallow entrapment or shrinkage bubble formation, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182–823) are most easily accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into H2O-rich melts a few days before eruption (e.g., Hartley et al., 2015). Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd = 51–216 versus 15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al+Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill diffusion: F is highly soluble in Al-rich melts. The high S/Dy of inclusions (300) indicates that primary melts were rich in S in comparison with most oceanic basalts. Although primitive plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series record few primary signals in their volatile element contents they nevertheless record information about crustal magma processing that is absent from olivine-hosted melt inclusions suites.